1. Field of the Invention
The present invention relates to a lithium polymer battery, and more particularly, to a battery unit configured to prevent a short circuit due to contact between the electrode leads and the electrode plates, a lithium polymer battery using the battery unit, and a method for manufacturing the lithium polymer battery.
2. Description of the Related Art
Lithium secondary batteries have a high energy density per unit weight and an operating voltage of 3.6V or greater, which is three times higher than nickel-cadmium (Ni—Cd) batteries, nickel-metal hydride (Ni-MH) batteries, and nickel-hydrogen batteries. For these reasons, their use has become widespread. Lithium secondary batteries can be classified into lithium-ion batteries that use a liquid organic electrolyte and lithium polymer batteries that use a solid polymeric electrolyte.
Specifically, lithium polymer batteries are rather safe and can accommodate a variety of shapes, especially, in thin film form to comply with the need for small, lightweight, portable electronic products. These properties could not typically be achieved in lithium-ion batteries. Due to these advantages, the lithium polymer battery has recently attracted attention.
Referring to FIGS. 1A and 1B, a conventional lithium polymer battery 10 is shown. The conventional lithium polymer battery 10 includes a battery unit 11, an electrode lead 12 drawn out from the battery unit 11, an electrode terminal 13 welded to the plurality of electrode leads 12, and a case 14 for accommodating the battery unit 11.
The battery unit 11 has a structure in which a cathode plate, an insulating separator, and an anode plate are sequentially and repeatedly stacked upon one another. The electrode lead 12 is drawn out from each of the cathode and anode plates of the battery unit 11. A plurality of electrode leads 12 drawn out from the electrode plates of the battery unit 11 are electrically connected to electrode terminals 13, wherein a portion of the electrode terminal 13 is exposed to the outside of the case 14. The case 14, shown in FIG. 1A as a pouch shape provides space for accommodating the battery unit 11.
The case 14 has a sealing portion 14a to be coupled to another element to seal the battery unit 11 placed in the case 14. A sealing tape 15 is wound around a portion of the electrode terminals 13 which contacts the sealing portion 14a. Accordingly, as the sealing portion 14a is thermally fused to seal the case 14, the sealing tape 15 is also fused and bound to internal layers of the case 14, thereby enhancing the hermetic containment of the battery.
However, the conventional lithium polymer battery 10 has the following problems. A group of electrode leads 12 extending from electrode plates, which have the same polarity as the electrode leads 12, is bent in a U-shape and then welded to one electrode terminal 13. When the insulating separator shrinks due to overheating during the manufacturing process or during operation of the lithium polymer battery 10, the electrode leads 12 may contact the electrode plates of the battery unit 11 having the opposite polarity to the electrode leads 12, thereby causing one or more short circuits.
In the case where the battery unit 11 has a stacked structure where the cathode and anode plates are stacked upon one another, sharp burrs may result at the edges of the electrode leads 12 in cutting a stack of the electrode plates to form the battery unit 11. The electrode leads 12 with such burrs may penetrate one of the insulating separators and directly contact electrode plates having the opposite polarity to the electrode leads 12, resulting in the occurrence of one or more short circuits in the battery.
Heat generated when a short circuit occurs in the battery unit 11 is transferred to a thin polymeric layer coated on the inner surface of the case 14. Accordingly, the thin polymeric film melts, and the electrode leads 12 electrically contact metal foil which is included as an intermediate structural layer of the case 14. As a result, the case 14 gradually corrodes depending on a difference in ionization with respect to the electrode plates of the battery unit 11.